Electrolyte for electrochemical device, method for preparing the electrolyte and electrochemical device including the electrolyte

ABSTRACT

Disclosed is an electrolyte for an electrochemical device. The electrolyte includes a composite of a plastic crystal matrix electrolyte doped with an ionic salt and a crosslinked polymer structure. The electrolyte has high ionic conductivity comparable to that of a liquid electrolyte due to the use of the plastic crystal, and high mechanical strength comparable to that of a solid electrolyte due to the introduction of the crosslinked polymer structure. Further disclosed is a method for preparing the electrolyte. The method does not essentially require the use of a solvent. Therefore, the electrolyte can be prepared in a simple manner by the method. The electrolyte is suitable for use in a cable-type battery whose shape is easy to change due to its high ionic conductivity and high mechanical strength.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of International Application No.PCT/KR2011/004337 filed on Jun. 14, 2011, which claims priority under 35USC 119(a) to Korean Patent Application Nos. 10-2010-0056062 and10-2011-0057343 filed in the Republic of Korea on Jun. 14, 2010 and Jun.14, 2011, respectively, the entire contents of which are incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure relates to an electrolyte for an electrochemicaldevice, a method for preparing the electrolyte, and an electrochemicaldevice including the electrolyte.

BACKGROUND ART

Secondary batteries, being the most representative of theelectrochemical devices, are devices which convert external electricalenergy to chemical energy, store the electrical energy and generateelectricity from the chemical energy when necessary. Secondarybatteries, or “rechargeable batteries”, are designed to be recharged andused multiple times. Lead-acid batteries, nickel cadmium (NiCd)batteries, nickel hydrogen (NiMH) batteries, lithium ion batteries andlithium ion polymer batteries are frequently used as secondarybatteries. Secondary batteries have lower costs of use and environmentalimpact than disposable primary batteries.

Secondary batteries are currently used in places where low power isneeded, for example, devices for assisting the start up of car engines,portable devices, instruments and uninterrupted power supply systems.The recent developments in wireless communication technologies have ledto the popularization of portable devices and have brought about atendency for devices to connect to wireless networks. As a result, thedemand for secondary batteries is growing explosively. In addition,hybrid vehicles and electric vehicles have been put into practical useto prevent environmental pollution, and by using secondary batteries inthese next-generation vehicles, they reduce the weight and cost andextend battery life for long-term use.

Generally, most secondary batteries are cylindrical, prismatic or pouchtype in shape because of their fabrication process. That is, a secondarybattery is typically fabricated by inserting an electrode assemblycomposed of an anode, a cathode and a separator into a cylindrical orprismatic metal can or a pouch type case made of an aluminum laminatesheet, and injecting an electrolyte into the electrode assembly.Accordingly, the cylindrical, prismatic or pouch type secondary batteryessentially requires a certain space for assembly, which is an obstacleto the development of various types of portable devices. Thus, there isa need for a novel type of secondary battery whose shape is easy tochange, and particularly, an electrolyte that has high ionicconductivity without any risk of leakage.

Ionically conductive organic electrolytes predominantly used forconventional electrochemical devices based on electrochemical reactionsare in the form of liquids in which salts are dissolved in non-aqueousorganic solvents. However, the use of such electrolytes in the form ofliquids causes degradation of electrode materials, increases thepossibility of evaporation of organic solvents, and poses safetyproblems, such as fire and explosion resulting from high surroundingtemperatures and increased battery temperatures. A risk of leakage anddifficulty in realizing various types of electrochemical devices areadditional safety problems. In attempts to overcome the safety problemsof such liquid electrolytes, polymer electrolytes, such as gel polymerelectrolytes and solid polymer electrolytes have been proposed. It isgenerally known that the safety of electrochemical devices increases inthe order of liquid electrolytes, gel polymer electrolytes and solidpolymer electrolytes, but the performance thereof decreases in the sameorder. It is known that electrochemical devices employing solid polymerelectrolytes are not yet commercialized due to these inferiorperformances. Gel polymer electrolytes have low ionic conductivity,suffer from the risk of leakage and possess poor mechanical propertiescompared to liquid electrolytes.

Korean Unexamined Patent Publication No. 2008-33421 discloses anelectrolyte using a plastic crystal matrix instead of using a liquidorganic solvent. The electrolyte exhibits ionic conductivity comparableto that of a liquid electrolyte. However, the electrolyte exhibits verypoor mechanical properties due to its flowability similar to that ofliquid. In actuality, a separator is required to prevent short circuitsin a battery using the electrolyte. In some cases, the introduction oflinear polymer matrices, such as polyethylene oxide, is considered toimprove the mechanical strength of plastic crystal matrix electrolytes.However, even in these cases, the electrolytes do not possess mechanicalproperties sufficient enough to replace the use of separators andbecause solvents are used, there exists a problem of having to add anadditional drying process.

Thus, there is an urgent need to develop a solid electrolyte using aplastic crystal matrix electrolyte that has improved mechanicalproperties while maintaining high ionic conductivity of the plasticcrystal matrix electrolyte.

DISCLOSURE Technical Problem

The present disclosure is designed to solve the problems of the priorart, and therefore it is an object of the present disclosure to providea plastic crystal matrix electrolyte that has high ionic conductivityand can ensure mechanical strength, and a method for preparing theelectrolyte.

Technical Solution

According to an aspect of the present disclosure, there is provided anelectrolyte for an electrochemical device which includes a composite ofa plastic crystal matrix electrolyte doped with an ionic salt and acrosslinked polymer structure.

The plastic crystal matrix may be, for example, succinonitrile.

The ionic salt is preferably a lithium salt. Examples of such lithiumsalts include lithium bis-trifluoromethanesulfonylimide, lithiumbis-perfluoroethylsulfonylimide and lithium tetrafluoroborate.

The crosslinked polymer structure may be obtained by polymerization of amonomer having two or more functional groups. The monomer having two ormore functional groups may be selected from trimethylolpropaneethoxylate triacrylate, polyethylene glycol dimethacrylate,trimethylolpropane trimethacrylate, ethoxylated bisphenol Adimethacrylate, and divinyl benzene.

Alternatively, the crosslinked polymer structure may be obtained bycopolymerization of the monomer having two or more functional groups anda monomer having one functional group. The monomer having one functionalgroup may be selected from methyl methacrylate, ethyl methacrylate,butyl methacrylate, methyl acrylate, butyl acrylate, ethylene glycolmethyl ether acrylate, ethylene glycol methyl ether methacrylate,acrylonitrile, vinyl acetate, vinyl chloride, and vinyl fluoride.

According to another aspect of the present disclosure, there is provideda method for preparing the electrolyte, the method including: adding amonomer having two or more functional groups to a plastic crystal matrixelectrolyte doped with an ionic salt to prepare a solution; andpolymerizing the monomer in the solution. Optionally, a monomer havingone functional group may be further added to the solution.

Advantageous Effects

The electrolyte of the present disclosure has high ionic conductivitycomparable to that of a liquid electrolyte due to the use of a plasticcrystal, and high mechanical strength comparable to that of a solidelectrolyte due to the introduction of a crosslinked polymer structure.In addition, the method of the present disclosure does not essentiallyrequire the use of a solvent, eliminating the need for drying.Therefore, the electrolyte of the present disclosure can be prepared ina simple manner. The electrolyte of the present disclosure is suitablefor use in a cable-type battery whose shape is easy to change due to itshigh ionic conductivity and high mechanical strength comparable to thatof a solid electrolyte.

DESCRIPTION OF DRAWINGS

The accompanying drawings illustrate preferred embodiments of thepresent disclosure and, together with the foregoing disclosure, serve toprovide further understanding of the technical spirit of the presentdisclosure. However, the present disclosure is not to be construed asbeing limited to the drawings.

FIG. 1 is a graph showing the electrochemical stability of electrolytesprepared in Example 1 and Comparative Example 2.

FIG. 2 is a graph showing the ionic conductivities of electrolytesprepared in Example 1 and Comparative Examples 1-2 with varyingtemperatures.

FIG. 3 is a graph showing the tensile strengths of electrolytes preparedin Example 1 and Comparative Example 2.

FIG. 4 is a graph showing the performance of half cells fabricated inFabrication Example 1 and Comparative Fabrication Example 1.

MODE FOR DISCLOSURE

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail with reference to the accompanying drawings. Itshould be understood that the terms used in the specification and theappended claims should not be construed as being limited to general anddictionary meanings, but interpreted based on the meanings and conceptscorresponding to technical aspects of the present disclosure on thebasis of the principle that the inventor is allowed to define termsappropriately for the best explanation.

The present disclosure provides an electrolyte for an electrochemicaldevice which includes a composite of a plastic crystal matrixelectrolyte doped with an ionic salt and a crosslinked polymerstructure.

The electrolyte of the present disclosure serves as a medium thattransports lithium ions between a cathode and an anode.

The plastic crystal is a compound whose molecules or ions exhibitrotational disorder but whose center of gravity occupies a positionaligned in the crystal lattice structure. The rotational phase of theplastic crystal is generally created by a solid-to-solid transition at atemperature not higher than the melting point. As a result of thesolid-to-solid transition, the plastic crystal exhibits high plasticity,mechanical flowability and conductivity. Particularly, the doping withan ionic salt results in high ionic conductivity, making the plasticcrystal suitable for use in an electrolyte for a secondary battery.However, flowability of the plastic crystal matrix electrolyte isdisadvantageous in terms of mechanical properties. For the purpose ofimproving this disadvantage, the crosslinked polymer structure isintroduced into the plastic crystal matrix electrolyte.

The crosslinked polymer structure has a three-dimensional structure dueto the chemical bonding between the molecular chains, unlike linearpolymers. This three-dimensional structure compensates for theflowability of the plastic crystal matrix electrolyte. In addition,since this crosslinking protects the crosslinked polymer structure fromthermal deformation, the electrolyte of the present disclosure does notsoften even when heat is applied thereto, ensuring thermal stability ofthe electrolyte.

The electrolyte of the present disclosure is a composite of the ionicsalt-doped plastic crystal matrix electrolyte and the crosslinkedpolymer structure. The composite may be prepared by homogenizing amonomer having two or more crosslinkable functional groups and the ionicsalt-doped plastic crystal matrix electrolyte, and polymerizing themonomer to form the crosslinked polymer structure. Alternatively, thecomposite may be prepared by homogenizing a monomer having two or morecrosslinkable functional groups, a monomer having one function group andthe ionic salt-doped plastic crystal matrix electrolyte, andpolymerizing the monomers to form the crosslinked polymer structure. Thecrosslinked polymer structure contributes to an improvement in themechanical properties of the electrolyte to impart the electrolyte withmechanical properties comparable to that of a solid electrolyte. Theuniform distribution of the plastic crystal matrix electrolyte increasesthe ionic conductivity of the electrolyte.

The weight ratio of the ionic salt-doped plastic crystal matrixelectrolyte to the crosslinked polymer structure may be from 30:70 to90:10.

It is preferred that the crosslinked polymer structure be obtained bypolymerization of the monomer having two or more functional groups or bycopolymerization of the monomer having two or more functional groups andthe monomer having one functional group. The monomer having two or morefunctional groups and the monomer having one functional group areintended to include not only monomers but also polymers with a lowdegree of polymerization consisting of 2 to 20 repeating units.

The kind of the monomer having two or more functional groups is notlimited. For example, the monomer having two or more functional groupsmay be selected from trimethylolpropane ethoxylate triacrylate,polyethylene glycol dimethacrylate, trimethylolpropane trimethacrylate,ethoxylated bisphenol A dimethacrylate and divinyl benzene.

The kind of the monomer having one functional group is not limited. Forexample, the monomer having one functional group may be selected frommethyl methacrylate, ethyl methacrylate, butyl methacrylate, methylacrylate, butyl acrylate, ethylene glycol methyl ether acrylate,ethylene glycol methyl ether methacrylate, acrylonitrile, vinyl acetate,vinyl chloride, and vinyl fluoride.

There is no restriction on the kind of the plastic crystal matrix.Succinonitrile is preferably used as the plastic crystal matrix.

The ionic salt doping the plastic crystal matrix electrolyte ispreferably a lithium salt. Examples of such lithium salts includelithium bis-trifluoromethanesulfonylimide, lithiumbis-perfluoroethylsulfonylimide and lithium tetrafluoroborate.

The present disclosure also provides a method for preparing theelectrolyte. Specifically, the electrolyte of the present disclosure isprepared by the following procedure.

First, a plastic crystal matrix electrolyte doped with an ionic salt ismixed with a monomer having two or more functional groups to prepare asolution (S1).

A monomer having one functional group may be optionally further added tothe solution.

Alternatively, an ionic salt, a plastic crystal matrix and a monomerhaving two or more crosslinkable functional groups may be mixed toprepare a solution. In this case, there is no need to previously preparethe ionic salt-doped plastic crystal matrix electrolyte.

The weight ratio of the crosslinked polymer structure to the monomer maybe from 30:70 to 90:10.

The monomer having two or more crosslinkable functional groups and themonomer having one functional group are intended to include not onlymonomers but also polymers with a low degree of polymerizationconsisting of 2 to 20 repeating units. The monomers may be selected fromabove-mentioned monomers. The plastic crystal matrix electrolyte may beany of the above-mentioned plastic crystal matrix electrolytes. Theionic salt may be any of the above-mentioned ionic salts. The ionic saltmay be used in an amount of 0.1 to 3 mole % per the plastic crystalmatrix.

A solvent may be added during mixing. In this case, drying isadditionally needed to remove the solvent. However, the use of thesolvent is not necessarily required. A photoinitiator, such as benzoin,may be optionally added to polymerize the monomer.

Subsequently, the monomer having two or more functional groups in thesolution is polymerized to prepare the solid electrolyte (S2).

There is no particular restriction on the polymerization method. Forexample, the monomer may be polymerized by UV irradiation. The presenceof two or more functional groups in the monomer allows the polymer tohave a three-dimensional crosslinked structure.

The present disclosure also provides an electrochemical device includinga cathode, an anode and the solid electrolyte. The electrochemicaldevice of the present disclosure includes all devices in whichelectrochemical reactions occur. Specific examples of suchelectrochemical devices include all kinds of primary batteries,secondary batteries, fuel cells, solar cells, and capacitors such assupercapacitor devices. Particularly preferred are lithium secondarybatteries, including lithium metal secondary batteries, lithium ionsecondary batteries, lithium polymer secondary batteries and lithium ionpolymer secondary batteries.

Particularly, the solid electrolyte of the present disclosure isinjected into an electrode assembly consisting of a cathode, an anodeand a separator interposed between the electrodes to fabricate a lithiumsecondary battery. The cathode, the anode and the separator constitutingthe electrode assembly may be those that are commonly used in thefabrication of lithium secondary batteries. The electrolyte of thepresent disclosure may replace the use of the separator because it is inthe form of a solid.

Each of the cathode and the anode is composed of an electrode currentcollector and an electrode active material. A lithium-containingtransition metal oxide is preferably used as an active material of thecathode. Specifically, the cathode active material may be selected fromthe group consisting of LiCoO₂, LiNiO₂, LiMnO₂, Li₂Mn₂O₄,Li(Ni_(a)Co_(b)Mn_(c))O₂ (0<a<1, 0<b<1, 0<c<1, a+b+c=1), LiNi_(1-y)CoA,LiCo_(1-y)Mn_(y)O₂, LiNi_(1-y)Mn_(y)O₂ (0<y<1), Li(Ni_(a)Co_(b)Mn_(c))O₄(0<a<2, 0<b<2, 0<c<2, a+b+c=2), LiMn_(2-z)Ni_(z)O₄, LiMn_(2-z)Co_(z)O₄(0<z<2), LiCoPO₄, LiFePO₄, and mixtures of two or more thereof. Otherexamples include sulfides, selenides and halides. The anode activematerial may be one capable of intercalating/deintercalating lithiumions. Examples of such anode active materials include carbon materials,lithium-containing titanium composite oxides (LTO); metals (Me), such asSi, Sn, Li, Zn, Mg, Cd, Ce, Ni and Fe; alloys of the metals (Me); oxidesof the metals (Me) (MeOx); and composites of the metals (Me) and carbon.Carbon materials are preferred. Low-crystalline carbon materials andhigh-crystalline carbon materials can be used. Representative examplesof low-crystalline carbon materials are soft carbon and hard carbon.Representative examples of high-crystalline carbon materials are naturalgraphite, Kish graphite, pyrolytic carbon, mesophase pitch based carbonfiber, meso-carbon microbeads, mesophase pitches, and high-temperaturesintered carbon materials, such as petroleum or coal tar pitch derivedcokes. The anode may include a binding agent. The binding agent may beselected from various kinds of binder polymers, such as vinylidenefluoride-hexafluoropropylene copolymers (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile and polymethylmethacrylate.

The separator may be a porous polymer film that is commonly used inseparators for lithium secondary batteries. Examples of materials forthe porous polymer film include polyolefin polymers, such as ethylenehomopolymers, propylene homopolymers, ethylene/butane copolymers,ethylene/hexane copolymers and ethylene/methacrylate copolymers. Theseparator may be a laminate of two or more porous polymer films. Theseparator may be a porous non-woven fabric. Examples of materials forthe porous non-woven fabric include, but are not limited to, highmelting-point glass fiber and polyethylene terephthalate fiber.

The shape of the lithium secondary battery according to the presentdisclosure is not particularly limited. The lithium secondary battery ofthe present disclosure may have a cylindrical or prismatic shapedepending on the shape of a can it uses. The lithium secondary batteryof the present disclosure may be of pouch or coin type. A cable typehaving a linear structure, such as a wire, is also possible.

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail. The embodiments of the present disclosure, however,may take several other forms, and the scope of the present disclosureshould not be construed as being limited to the following examples. Theembodiments of the present disclosure are provided to more fully explainthe present disclosure to those having ordinary knowledge in the art towhich the present disclosure pertains.

EXAMPLES Example 1 Preparation of Crosslinked Polymer Structure/PlasticCrystal (15/85) Electrolyte

Lithium bis-trifluoromethanesulfonylimide was added to succinonitrile toprepare a 1 M plastic crystal electrolyte. Trimethylolpropane ethoxylatetriacrylate (TMPEOTA) and the plastic crystal electrolyte werehomogenized in a weight ratio of 15:85. Benzoin as a UV initiator, in anamount of 3 wt %, was added to the mixture, based on the weight of theTMPEOTA.

Thereafter, the resulting mixture was cast on a glass plate andirradiated with UV for 20 sec. As a result of the polymerization, anelectrolyte was produced in the form of a membrane.

Example 2 Preparation of Crosslinked Polymer Structure/Plastic Crystal(30/70) Electrolyte

An electrolyte membrane was produced in the same manner as in Example 1,except that the mixing ratio of trimethylolpropane ethoxylatetriacrylate (TMPEOTA) and the plastic crystal electrolyte were mixed ina weight ratio of 30:70.

Example 3 Preparation of Crosslinked Polymer Structure/Plastic Crystal(50/50) Electrolyte

An electrolyte membrane was produced in the same manner as in Example 1,except that the mixing ratio of trimethylolpropane ethoxylatetriacrylate (TMPEOTA) and the plastic crystal electrolyte were mixed ina weight ratio of 50:50.

Comparative Example 1 Preparation of Pure Plastic Crystal MatrixElectrolyte

Lithium bis-trifluoromethanesulfonylimide was added to succinonitrile toprepare a 1 M plastic crystal matrix electrolyte in a pure form.

Comparative Example 2 Preparation of Linear Polymer Matrix CrystalElectrolyte

Lithium bis-trifluoromethanesulfonylimide was added to succinonitrile toprepare a 1 M plastic crystal electrolyte. PVdF-HFP and the plasticcrystal electrolyte in a weight ratio of 15:85 were homogenized inacetone as a solvent. The solvent was used in an amount of 20 wt %,based on the total weight of the mixture.

After the mixture was cast on a glass plate, the solvent was removed byevaporation in a glove box under an argon gas atmosphere to prepare anelectrolyte membrane in the form of a solid.

Fabrication Example 1 Fabrication of Half Cell

Each of the electrolyte membranes produced in Examples 1-3 was insertedbetween tin-plated copper as a working electrode and lithium metal as acounter electrode to fabricate a coin-type half cell.

Comparative Fabrication Example 1 Fabrication of Half Cell

A polyethylene separator was interposed between tin-plated copper as aworking electrode and lithium metal as a counter electrode to constructan electrode assembly. Thereafter, an electrolyte solution of 1 M LiPF₆in a mixture of ethylene carbonate and diethyl carbonate (1:2, v/v) asnon-aqueous solvents was injected into the electrode assembly tofabricate a coin-type half cell.

Test Example 1 Measurement of Electrochemical Stability

The electrolytes prepared in Example 1 and Comparative Example 2 weremeasured for electrochemical stability. The results are shown in FIG. 1.For the measurement, each of the electrolyte membranes produced inExample 1 and Comparative Example 2 was inserted between stainless steelas a working electrode and lithium metal as a counter electrode tofabricate a coin-type half cell. The electrochemical stability of thecoin-type half cell was measured with increasing voltage to 6 V at ascan rate of 5 mV/s by linear sweep voltammetry (LSV).

As can be seen from FIG. 1, the electrolyte of Example 1 including thecrosslinked polymer structure and the plastic crystal electrolyte showsimproved electrochemical stability, compared to the electrolyte ofComparative Example 2 including the linear polymer matrix. Particularly,the electrolyte of Example 1 is electrochemically stable up to 5 V.

Test Example 2 Measurement of Ionic Conductivities Depending on theContent of Crosslinked Polymer Structure

The ionic conductivities of the electrolytes of Examples 1-3 havingdifferent contents of the crosslinked polymer structure were measured.The results are shown in Table 1.

TABLE 1 Plastic crystal Ionic conductivity solid electrolyte (25° C.,S/cm) Example 1 2.4 × 10⁻³ Example 2 2.3 × 10⁻⁴ Example 3 5.1 × 10⁻⁵

Test Example 3 Measurement of Ionic Conductivities with VaryingTemperatures

The ionic conductivities of the electrolytes of Example 1 andComparative Examples 1-2 were measured with increasing temperature from30° C. to 70° C. The results are shown in FIG. 2.

As can be seen from FIG. 2, the ionic conductivities of the electrolytesincrease in proportion to the temperature. Particularly, the ionicconductivity of the electrolyte of Example 1 including the crosslinkedpolymer structure is slightly lower than that of the linear polymermatrix plastic crystal electrolyte of Comparative Example 2 but ishigher than that of the pure plastic crystal electrolyte of ComparativeExample 1.

Test Example 4 Measurement of Mechanical Properties

The tensile strengths of the electrolytes prepared in Example 1 andComparative Example 2 were measured. The results are shown in FIG. 3.Referring to FIG. 3, the electrolyte of Example 1 including thecrosslinked polymer structure shows greatly improved physical propertiescompared to the polymer matrix plastic crystal electrolyte ofComparative Example 2.

Test Example 5 Charge-Discharge Tests on Half Cells

Each of the half cells fabricated in Fabrication Example 1 andComparative Fabrication Example 1 was charged to 5 mV with a currentdensity of 0.5 C under constant current conditions and maintained at aconstant voltage of 5 mV. The charging was stopped when the currentdensity reached 0.005 C. The half cell was discharged to 1.5 V with acurrent density of 0.1 C in a CC mode. Charge and discharge cycles wererepeated under the same conditions. The normalized graph is shown inFIG. 4.

The half cell of Fabrication Example 1 has a slightly higher resistancethan the half cell of Comparative Fabrication Example 1 including theliquid electrolyte solution and the separator, but its performance iscomparable to that of general half cells.

1.-8. (canceled)
 9. A method for preparing a solid electrolyte,comprising: (S1) mixing a plastic crystal matrix electrolyte doped withan ionic salt and a monomer having two or more functional groups toprepare a solution; and (S2) polymerizing the monomer in the solution.10. The method according to claim 9, wherein the solution furthercomprises a monomer having one functional group.
 11. The methodaccording to claim 9, wherein the ionic salt-doped plastic crystalmatrix electrolyte and the monomer are used in a weight ratio of 30:70to 90:10.
 12. The method according to claim 9, wherein the plasticcrystal matrix electrolyte comprises succinonitrile.
 13. The methodaccording to claim 9, wherein the ionic salt is used in an amount of 0.1to 3 moles per mole of the plastic crystal matrix electrolyte.
 14. Themethod according to claim 9, wherein the ionic salt is a lithium salt.15. The method according to claim 14, wherein the lithium salt isselected from lithium bis-trifluoromethanesulfonylimide, lithiumbis-perfluoroethylsulfonylimide, lithium tetrafluoroborate, and mixturesthereof.
 16. The method according to claim 9, wherein the monomer havingtwo or more functional groups is selected from trimethylolpropaneethoxylate triacrylate, polyethylene glycol dimethacrylate,trimethylolpropane trimethacrylate, ethoxylated bisphenol Adimethacrylate, and mixtures thereof.
 17. The method according to claim10, wherein the monomer having one functional group is selected frommethyl methacrylate, ethyl methacrylate, butyl methacrylate, methylacrylate, butyl acrylate, ethylene glycol methyl ether acrylate,ethylene glycol methyl ether methacrylate, acrylonitrile, vinyl acetate,vinyl chloride, vinyl fluoride, and mixtures thereof. 18.-19. (canceled)